gnupatch: add patch for CVE-2018-6952

Refs:
https://nvd.nist.gov/vuln/detail/CVE-2018-6952
https://savannah.gnu.org/bugs/index.php?53133

pkgs/tools/text/gnupatch/CVE-2018-6952.patch

···
+From 9c986353e420ead6e706262bf204d6e03322c300 Mon Sep 17 00:00:00 2001
+From: Andreas Gruenbacher <[e-mail]>
+Date: Fri, 17 Aug 2018 13:35:40 +0200
+Subject: Fix swapping fake lines in pch_swap
+
+* src/pch.c (pch_swap): Fix swapping p_bfake and p_efake when there is a
+blank line in the middle of a context-diff hunk: that empty line stays
+in the middle of the hunk and isn't swapped.
+
+Fixes: https://savannah.gnu.org/bugs/index.php?53133
+---
+ src/pch.c | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/src/pch.c b/src/pch.c
+index e92bc64..a500ad9 100644
+--- a/src/pch.c
++++ b/src/pch.c
+@@ -2122,7 +2122,7 @@ pch_swap (void)
+     }
+     if (p_efake >= 0) {			/* fix non-freeable ptr range */
+ 	if (p_efake <= i)
+-	    n = p_end - i + 1;
++	    n = p_end - p_ptrn_lines;
+ 	else
+ 	    n = -i;
+ 	p_efake += n;
+-- 

pkgs/tools/text/gnupatch/default.nix

···
       url = https://sources.debian.org/data/main/p/patch/2.7.6-2/debian/patches/Fix_arbitrary_command_execution_in_ed-style_patches.patch;
       sha256 = "1bpy16n3hm5nv9xkrn6c4wglzsdzj3ss1biq16w9kfv48p4hx2vg";
     })
+    # https://git.savannah.gnu.org/cgit/patch.git/commit/?id=9c986353e420ead6e706262bf204d6e03322c300
+    ./CVE-2018-6952.patch
   ];
 
   nativeBuildInputs = [ autoreconfHook ];